1. Technical Field
This invention relates to a cipher communication method and device suitable for digital data communication, and relates in particular to a cipher communication method and device that can be used even when there are gaps in the communication data on the transmission pathway.
2. Background Art
Communication information encryption is performed in digital data communication in order to protect communication from wire tapping. An example is shown in FIG. 8(A). Bit strings (open information) that have not yet been encrypted are input from a terminal 41, and encrypted bit strings (encrypted information) are output from a terminal 42. 45 is a 13-bit shift register in which the bit string that is to serve as the key for encryption is stored in advance. Every time a single bit of data is input to an adder 43 from the terminal 41, the bit string of the shift register 45 shifts one bit in the direction of the arrow 49, and the single bit of data is input to the adder 43.
The adder 43 calculates (exclusive-OR) the data input from the terminal 41 and the shift register 45, and outputs the result from the terminal 42 as encrypted information. The most significant bit 45M data of the shift register 45 is input to the adder 43, as well as to the least significant bit 45A and an adder 46 at a specified position of the shift register 45. The contents of the shift register 45 are the same for every 2.sup.12 -1 shift cycle.
Decoding (decryption) is performed in the reverse order from encryption, as shown in FIG. 8(B). An encrypted bit string input from the terminal 51 is input along with the bit string of the shift register 55 to the adder 53. The calculation results of the adder 53 are output from the terminal 52 as decoded bit strings. The shift register 55 already contains the same bit strings (keys) as the shift register 45, and every time there is a shift the most significant bit 55M data of the shift register 55 is input to the adder 53, as well as to the least significant bit 55A and an adder 56 at a specified position of the shift register 55.
With the above method, however, when gaps occur in the bit strings on the transmission pathway during decoding, the bit strings of the shift register 56 are synchronized and shifted with the incomplete bit strings. Discrepancies occur in the encrypted bit strings following the occurrence of the gaps, and in the encryption keys (the bit strings of the shift register 56), and this renders decoding impossible. If a gap occurs in a bit string on the transmission pathway, the subsequent encrypted bit strings will not be able to be decoded.
There are various communication procedures for digital data communication which include error correction functions for correcting transmission errors on the transmission pathway, and error correction functions for detecting missing data and executing retransmission. In a communication procedure equipped with an error correction function that can restore missing data on the transmission pathway, an encryption method such as that described above can be used, but either more transmission time is required for retransmission, or more sophisticated encryption and decoding processing is required.